Images contain and consist of a lot of data. It takes a long time to download large images through a network and this causes undesired waiting times. Download of images is often performed when surfing the Internet or when in other ways connected to a network of computers. The waiting times will be considerably longer if using a slow transmission link, such as being connected to the Internet through a modem. These waiting times are so long today that many users will not even bother to try to download a large image when using a slow transmission link.
In order to reduce the waiting times of downloading, it is known to compress the images so that the amount of data that is downloaded is reduced. However, image compression could only be performed up to a certain limit, then image information is being lost. An image that has been compressed sufficiently to allow short downloading times will then not show enough details.
Attempts have been made to solve this problem in several ways. One way is to present a small image, which has been hugely compressed. The user could then choose if he wants to download the original, large image by clicking on the image to get the download. This download takes time but is not forced on the user.
Another method is disclosed by Talagala et al., Usage Patterns of a Web-Based Image Collection, IEEE Symposium on Mass Storage Systems 1999: 203-214. According to this method an image is stored in different magnitudes on a server. When a user accesses a web site, the image is first presented in the smallest magnitude and a user could click to zoom in the image. The image is then downloaded in a larger magnitude and presented to the user. If the image size exceeds an image frame in the web site, the area in which the user clicked is presented and the image could be panned to show other parts of it. The image is divided into smaller tiles, which are downloaded in sequence, and so the image is presented bit by bit during downloading.
According to these methods a large image is not presented until the user explicitly asks for it. However, when a user wants to see an image in larger detail, a large image is downloaded causing waiting times. The small tiles, in the method according to Talagala et al., make the downloading time of an individual tile relatively short, but the header of the image file is rather large in order to control where each tile should be presented in the image. The user sees the image being built up bit by bit by the tiles, but has to wait for a while before all tiles have been downloaded.